Effect Of Shell Materials And Shell Thickness Of ZnCdSe Gradient Alloy Quantum Dots On The Performance Of Light-Emitting Diodes

In recent years,quantum dots（QDs） have attracted wide attention due to their high photoluminescence quantum yields（PL QYs）,good photostability,wide absorption spectrum,narrow emission spectrum and continuous tunable spectrum.QDs are widely used in the field of light emitting diodes（QLEDs）due to their solution processability and simple operation.Since the first report of QDs-based QLEDs in 1994,researchers have improved the performance of QDs by improving the performance of QDs,optimizing the device structure and charge transfer layer materials and so on.For more than two decades,although the performance of QLEDs has been greatly improved,the low carrier injection efficiency and the imbalance of carrier injection still restrict the performance of the devices.When the device on operation,electrons and holes need to pass through the shell layer of QDs to form exciton recombination luminescence in the core.Therefore,the shell layer has a great influence on the efficiency of carrier injection.On the one hand,due to the HOMO energy level of the organic hole transport layer is much higher than the valence band energy level of the QDs,so the hole injection barrier is high.This will lead to the imbalance of carrier injection and the aggregation of carriers at the barrier interface.So the Auger recombination process will occur,which will lead to the increase of driving voltage and efficiency roll-off.Therefore,the selection of appropriate shell material can reduce the hole injection barrier,promote the carrier injection balance,and then improve the performance of the device.On the other hand,the thickness of the shell of QDs also has a great effect on the performance of the device.Under the action of electric field,the exciton separation is easily caused by the QDs in the thin shell layer,and the injection efficiency of the carriers is reduced by the QDs in the thick shell layer.Based on the above analysis,In this paper,we studied the influence of gradient alloy structure,shell material and shell thickness of QDs based on the ZnCdSe core on the performance of QLEDs,the main contents of this paper include the following two aspects:（1）Synthesis of Zn_1-xCd_xSe gradient alloy QDs and ZnCdSe/ZnS core/shell structure QDs and the fabrication of QLEDs:First,by controlling the ratio of Cd to Zn,a series of Zn_1-xCd_xSe gradient alloy QDs with different emission wavelengths are synthesized by a thermal injection method.A series of ZnS shells of different thickness were grown on the Zn_1-xCd_xSe gradient alloy core,and the QYs reached 92%when the thickness of the ZnS shell is 8 monolayers（monolayer,ML）,and the photochemical stability is good.The single-particle fluorescence data show that the time of light state is more than 95%of ZnCdSe/ZnS core/shell QDs with ZnS shell thickness of 5 ML,8 ML and 10 ML,which is non-blinking QDs.Finally,the synthesized QDs was applied to the QLEDs,and the effect of ZnS shell material and thickness on the performance of the light-emitting diode was studied.By using Zn_1-xCd_xSe gradient alloy QDs as emitter in QLEDs,the maximum external quantum efficiency is 8.5%,and the maximum brightness is 155,330 cd/m².Next,the maximum external quantum efficiency based on the ZnCdSe/ZnS core/shell QDs is up to 14.7%,and the maximum brightness is 258,998 cd/m².（2）Synthesis of ZnCdSe/ZnSe and ZnCdSe/ZnSe/ZnS core/shell QDs and the fabrication of QLEDs:ZnSe shell and ZnS outer shell layer were grown on ZnCdSe gradient alloy core by continuous ion layer adsorption method.Ultraviolet-visible absorption spectrum and photoluminescence spectrum data show that the peak positions are blue-shifted with the increase of shell thickness,which indicates that the alloy interface layer is formed between the core and shell of QDs at high temperature.The PL QYs of all QDs are more than 90%,and ZnCdSe/ZnSe/ZnS QDs has better photochemical stability.Finally,the synthesized QDs was applied to the QLEDs,the results show that the maximum external quantum efficiency is 20.5%of the QLEDs based on ZnCdSe/ZnSe core/shell QDs,the maximum brightness is 460,200 cd/m²,and the efficiency roll-off is low.The maximum external quantum efficiency reached as high as 22.0%of the device based on ZnCdSe/ZnSe/ZnS core/shell QDs,the maximum brightness is 283,930 cd/m²,and the efficiency roll-off is also very low.The excellent performance of the device is mainly attributed to the growth of ZnSe shell.ZnSe shell can reduce the hole injection barrier and promote the balance of carrier injection,and then improve the efficiency of the device.Moreover,ZnS outer shell can not only improve the stability of QDs,but also does not affect the charge injection.